Announcing method and system



Jan. 1, 1946. 1.. J. ANDERSON ,392, 1

ANNOUNCING METHOD AND SYSTEM Filed Jan. 9, 1945 MWRVQ N NINQU QNEE H mmmm R an INVENTOR. my I :1

BY v

FTTOE/Vfy 2- sesame I I Y and above 6500 cycles per second. It has beenon a transformer It over condenser it. The

regular grid biasing resistors 20 and 21 shunted by bypass condensers 22and 28 are shown in the cathode circuits of the respective tubes to andI1. Anode potential for tube i! is supplied over resistor 2d.

The signal amplified in amplifier I1 is -reproduced by one or more loudspeakers such as I The transformer is does shown at'fit and 21.

' not directly feed the speakers 26 anad El, these elements beinginterconnected by a bandpass filter 2t composed of two series resonant,series connected sections and a parallel resonant shunt 'the level ofthe signal channel in substantially direct proportion'to the increase innoise in the found that'this proportioning of the frequency bands willpermit the control circuit to raise sound reproducing area, whilepreventing feedback in the system.

connected section. One series section comprises a condenser St andinductance 3i, and the other series section comprises an inductance $2and a condenser 33. The parallel shunt element is composed of aninductance rs and a condenser as. and is connected from a point betweenthe series sections and the other side of the line.

Located in the area in which sound is reproduced by the speakers at and2? is a control pickup microphone dd which is connected to an amplifiertube ti through a transformer t2 and a band elimination filter dd. Thisband elimination filter is composed of two parallel resonant sectionsconnected in series, one section being vcomposed of an inductance 65 anda condenser fil and the other section being composed of i an inductancees and a condenser fit. To complete this filter, a series resonantsection, composed of a condenser E36 and an inductance 52, is connectedfrom a point between the parallel resonant sections and the other sideof the line.

The output of amplifier ill is impressed on a full wave rectifiercomposed of vacuum tube rectifiers 55 and 56 over a transformer El, the

output being passed through a capacity resistance filter to, a loadresistor 50. The capacityresistance filter is composed of threeresistors 6 l,

62, and $3, and two condensers 6t and st, these elements being givenvalues to provide the proper timing desired during increases anddecreases in the control signal.

It will be noted that the control grid of variable gain amplifier I t isconnected through the secondary of transformer 9 and the rectifier loadresistor $0 andback to the cathode of tube id through fixed biasresistor 20.

In this. manner, variations in current through the resistor 60 will varythe bias on tube l0 and thus vary its gain. The circuit is so arrangedthat increases in the amplitude of the signal detected by the microphone40 will reduce the bias on tube it to increase its gain, so that theoutput of-the signal channel at speakers 26 and 21 will be increasedproportionately.

To prevent the output of the speakers 26 and 21 from being detected bymicrophone 40 and thus cause regeneration. feedback, or singing, the twofilters 2t and M are employed. Referring now to Fig. 2, thecharacteristics of the two filters are illustrated, filter 28 being aband pass filter passing frequencies from approximately- 400 to6500cycles per second. This frequency range is one which will provide thesignal with As mentioned above, other signal channels may feed otherlistening areas as indicatedby the dotted lines 8, these other channelseither having similar automatic level control circuits in case thespeakers are in noise areas, or having .solely an amplifier or fixedgain in case the sound is reproducedin silent areas, such as omces orsimilar locations. I

I claim as my invention:

1. In a system including an amplifier connected between a signal sourceand a reproduclog device, the combination of a band pass filterconnected between said amplifier and saidrelying in a predetermined handbetween substantially 400 and 6500 cycles per second, a pickup device, arectifier, a band elimination filter for restricting the output fromsaid pickup device as impressed on said rectifier to impulses of fre-'quencies lying below 400 and above 6500 cycles per second, detectingsound frequencies in said I per second, and means for directlycontrolling the bias on said amplifier-with the output of saidrectifier. v

2. The method of maintaining a substantially constant signal to noiseratio 'in a listening area served by a sound reproducer and subject tovarying noise levels comprising selecting for reproduction apredetermined band of frequencies in said signal lying between 400 and6500 cycles area lying below 400'and above 6500 cycles per second, anddirectly controlling the level of reproduction of said signalfrequencies by the amplitude of said detected sound frequencies.

3. The method of increasingthe level of reproduction of a signal as thenoise level in the reproduction area increases without causingregeneration comprising reproducing only a predetermined band offrequencies in saidlsignal lying between 400 and 6500 cycles per second,

detecting only the sound frequencies lying be-,

low 400 and above- 6500 cycles per second, and increasing the level ofsignal reproduction directly in accordance with the amplitude of theproducts of detection. p

4. An announcing system comprising a sound pickup device, an amplifierfor the output of said device, a reprdducer for translating-the outputof said amplifier into sound waves, a band pass filter connected betweensaid reproducer and amplifier for suppressing the impression offrequencies lying below 400 and above 6500'cycles per second in theoutput of said device on said reproducer, a second sound pickup deviceacoustically coupled to said reproducer, a rectifier for the output ofsaid second device, a band elimination filter connected between saidrectifier and said second pickup device for suppressing the imgoodintelligibility for speech transmission and.

dotted lines, while passing frequencies below 400 pression offrequencies lying between 400 and 6500 cycles per second on saidrectifier, and

means for connecting said rectifier to said amplifier for directly.controlling the gain of said amplifier in accordance-"with thgamplitudeof the output of said rectifier.

. LESLIE J. ANDERSON.

